whereas the Magothy beds are regarded as marine deposits. The intermingling of clay, sand 

 and gravel, observed in the Magothy beneath Long Island suggests that here also it may be ter- 

 restrial. None of the Cretaceous layers show the continuity of structure or composition that 

 characterize most marine sediments, and most marine sediments of Cretaceous age contain 

 abundant fossils. 



This lack of continuity in the Cretaceous beds is of considerable importance hydrologically. 

 In the first place it is not possible to predict in advance at what depth water will be available, 

 nor in what quantity. It also increases the uncertainty as to the quality of the water, and makes 

 it much more difficult to determine the safe yield of any well, or the danger of salt water intru- 

 sion into a well field. By and large it is necessary to treat the Magothy as a single hydrologic 

 unit since the occurrence and movement of water in it is not known in any detail, except very lo- 

 cally. A better idea of its mode of origin would make possible a far better understanding of 

 many of its hydrologic problems. 



The oldest of the Cretaceous deposits on Long Island is the Lloyd sand member of the Rari- 

 tan formation. How long a time was required for its deposition is not known, but the coarse 

 sand and pebbles which form much of the Lloyd suggest fairly rapid deposition by swiftly 

 moving streams or currents. However, conditions were not entirely constant throughout its 

 formation, for there are locally one, two or more layers of clay interbedded with the layers of 

 sand and gravel. These changes in the nature of the material deposited were probably not caused 

 by changes in sea level. It is more likely that they were the result of shifts in the positions of 

 the streams which deposited the sediments, and the clay beds may therefore be local in extent 

 rather than regional. That they are indeed not extensive is shown by the fact that the with- 

 drawal of water from one bed of the Lloyd sand causes a lowering of the water pressure in all 

 the other beds of the Lloyd regardless of any intervening layers of clay. In other words the 

 Lloyd sand in some places at least is hydrologically a unit. 



One consistent variation shown by the Lloyd is an increase in its thickness from northwest 

 to southeast. This increase in thickness down the dip is characteristic of all the Cretaceous de- 

 posits along the Atlantic coastal plain. 



The generally eastward tilting of the Fall Zone peneplain away from the axis of uplift 

 lowered the outer parts of the peneplain more than the inner, and there was consequently a 

 greater accumulation of sediment in those areas which sank the most. As any one unit is traced 

 inland, it becomes thinner until it finally wedges out. The next overlying unit however overlaps 

 the preceding one and extends still further inland, showing that the sinking of the outer por- 

 tions of the peneplain was not a simple single movement, which took place in a limited time, and 

 then was over and done with. Rather it was a very slow but generally steady process, in which, 

 as the geologist commonly finds, sinking more or less kept pace with deposition. 



The original landward limit of the Lloyd sand member of the Raritan formation is 

 suggested by its apparent wedging out in central Kings and Queens Counties. Following the 

 general pattern of Cretaceous deposition, the clay member of the Raritan, which overlies the 

 Lloyd sand, overlapped the Lloyd and reached an unknown distance farther north. The still 

 higher so-called Magothy formation probably extended still farther inland than the clay member 

 of the Raritan. However, subsequent erosion of both the Magothy and the Raritan clay has 

 removed so much material that their northern limit today, in a general way, is the northern 

 shore of Long Island. 



Presumably if the lower contact of the Lloyd sand were followed down the dip far enough 

 to the southeast another group of Cretaceous beds older than the Lloyd would appear between 

 the Lloyd sand and the bedrock. These pre-Lloyd beds have not been found in any of the wells 

 drilled on Long Island. There is, however, a slight chance that older Cretaceous beds exist 

 beneath the Lloyd in south central Suffolk County. 



33 



